I am considering using this extension for my future Joomla sites.

If I get two mirror sites going with this extension. Is it possible to configurate one site out of reach of public? So I can use it as testing ground for my master site?

I need to find a solution for a testing environment. Can anyone tell me if this is right tool?

Thank you in advance.

Yi

I have some customer that already use JMS to manage development site and production site.
This is totally possible when you want to create themes (template) and have different layout.

If you objective is to modify component, modules or plugin PHP code, I just would like to remember you that you share the same PHP files and folders on the disk.

With JMS, you can perfectly create several slave sites that you are copy of a production website and that you use to test layout, setup, ...
As you have a status on the slave site, you can easily switch from one website to another one in production.
You can for example, for a same domain name have a website slave_vers1 in production and prepare a slave_vers2 that you use in development. When it is OK, you can disable slave_vers1 to switch in slave_vers2.

All I can tell you is that I have at least one customer that use JMS for his development and production